A novel class of repeated genes in Drosophila melanogaster has been described in which the elements are widely dispersed throughout the genome rather than tandemly arranged (Finnegan, Rubin, Young, and Hogness, 1978, Cold Spring Harbor Symp. Quant. Biol. 42: 1053-1063). Recent evidence from this laboratory suggests that the elements of these gene families are mobile within the genome and undergo significant rearrangement in both fly populations and tissue culture cell lines. Two dispersed repeated gene families, 412 and copia, have been studied in detail and exhibit very similar patterns of sequence organization although they show no sequence homology. We plan to study the mechanism(s) responsible for the observed transposition of elements using the techniques of molecular cloning. We will isolate several cloned segments of Drosophila melanogaster tissue culture DNA which contain an inserted element of a gene family, and then use single copy sequences which are adjacent to the elements on these DNA segments as probes to screen a library of cloned embryonic DNA sequences. We will thereby isolate pairs of DNA segments from tissue culture and embryo cells which are identical except that the tissue culture cell DNA will have had an element of a dispersed gene family inserted. By comparing the structure of such pairs of DNA segments we will be able to distinguish several possible mechanisms for their movement within the genome.